Guns of August
Let us assume the opening moves of the Great War in August 1914 go as planned for the Germans for a few more weeks; i.e., no turn north and east of Paris presenting a flank to be attacked which resulted in stopping the German Army and the formation of the trench line across northern France. So the German Army approaches Paris from the north and east.
What happens now? Do the Germans have sufficient manpower and logistics to attack or invest Paris? Can they lay siege and continue on or will that stop their advance? Does the BEF keep contact with the French forces during the retreat or do they keep one flank on the sea and allow a gap to open between them and the French Army? Do the French fight for Paris or declare it an open city and allow the Germans to enter? Does the French government stay or flee to somewhere else? Does the war continue, or do the French begin to seek an armistice? Will the result be a Sedan-like battle with the French Army abandoning the Maginoit fortresses to mass in the field (and who would win), or would it be the same old war with the trenches further south and west? If so, how long will it last now, and who has the advantage over OTL?
I'm sure we have some writers here who can make Barbara Tuchman proud.